Thz Low Resolution Spectroscopy for Astronomy Gordon J

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IEEE TRANSACTIONS ON TERAHERTZ SCIENCE AND TECHNOLOGY, VOL. 1, NO. 1, SEPTEMBER 2011 241 THz Low Resolution Spectroscopy for Astronomy Gordon J. Stacey

(Invited Paper)

Abstract—The THz spectral regime provides a wide range of new astrophysical discoveries, Section IV is a discussion of the spectral lines that are invaluable probes of star formation and merits of direct detection systems, Section V gives examples of AGN activity in galaxies both in the local Universe and at the moderate resolving power direct detection systems that are used earliest times. We review the utility of these lines, give examples of the science they deliver, and detail the properties of successful for astrophysical experiments today, and Section VI discusses low resolution direct detection spectrometers for work in the THz some of the prospects for future work. regime. We finish with a discussion of the exciting new science we expect with the next direct detection generation spectrometers on II. ASTROPHYSICAL PROBES WITHIN THE THZ BANDS new facilities such as SOFIA, CCAT, SPICA, and ALMA. A. Fine-Structure Lines Index Terms—Direct detection receivers, extragalactic spectroscopy, fine-structure lines, TH spectroscopy. Atoms or ions with ground state electronic configurations that contain valence electrons will have their configurations ordered by the residual electrostatic interaction into terms that differ in I. INTRODUCTION the total (vector) orbital angular momentum and HE THz spectrum, here loosely defined as frequencies total (vector) spin angular momentum .Theseterms T between 600 GHz and 6 THz, contains a plethora of are donated by , where the superscript is equivalent readily detectable spectral lines that are important diagnostics to the number of levels into which the term is split, and P de- for both the physical and chemical conditions of the gas and notes the total orbital angular momentum of the electronic term the sources of energy within astrophysical environments. These and follows the usual convention where refer to spectral probes include rotational lines from simple molecules terms of . These terms are further split by the (e.g., HD, CH, OH, CO, , ), and the ground state spin-orbit interaction into levels where the subscript J fine-structure lines from abundant atoms and ions (e.g., C, denotes the magnitude of the vector sum of and . , , ,O,and ). These lines are well explored in Each of these levels, J, has a degeneracy given by . both galactic and extragalactic environments, and indeed the When an atom or ion has 1, 2, 4, or 5 equivalent p electrons in molecular lines can be important probes of planetary atmo- its ground state configuration, the ordering and selection rules spheres in the solar system. Important results are obtained with dictate that its ground state term will be or terms, so that both coherent (heterodyne) and incoherent (direct detection) it has ground state fine-structure lines. After H and He, which spectrometers. Here we focus on work done with direct detec- have no significant lines in the THz regime, the three most abun- tion spectrometers whose natural niche is the broad spectral dant elements in the Universe are oxygen, carbon, and nitrogen line environment of extragalactic studies. In this context, line with relative abundances ,3,and with respect to widths are to 500 km/sec, the smaller values typical for hydrogen respectively. Amongst these elements, the O, , low mass dwarf galaxies and spiral galaxies viewed “face-on” C, , ,and ionization states have ground term split and the larger values more typical for more massive systems into fine-structure levels that emit photons in the THz range with more arbitrary presentations on the sky. This narrows our (Table I). Since transitions within a term involve no change focus to the lines with large luminosities on galactic scales. in the electronic configuration, they are forbidden to electronic These are the lines that are important coolants of the interstellar dipole radiation and decay by magnetic dipole transitions. As medium (ISM) including the fine-structure lines of C, , , such, the excited fine-structure levels are metastable with life- ,O,and , and the mid-J rotational lines of CO. times measured in days. This paper is divided into five sections. Following this intro- Since they lie in the THz range, where the corresponding duction, Section II discusses these spectral lines and their utility wavelengths 100 m are large compared to the typical sizes as probes of astrophysical environments, Section III is a brief of interstellar dust grains 0.1 m , these lines are largely history of observations of these lines including some exciting unaffected by the interstellar extinction by grains that hinders optical and near infrared observations. With weak Einstein A coefficients, the lines are also not usually affected by self-ab- Manuscript received April 08, 2011; revised June 06, 2011; accepted June 07, sorption (they are optically thin) so that typical photons will es- 2011. Date of current version August 31, 2011. This work was supported in part by NSF Grants AST-0705256 and AST-0722220. cape an emitting gas cloud. Furthermore, the energy levels that The author is with the Department of Astronomy, Cornell University, Ithaca, emit the lines lie within a few hundred K of the ground state, so NY 14850 USA (e-mail: [email protected]). that they are easily collisionally excited by electrons, H, or Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. impacts in many astrophysical environments. The combination Digital Object Identifier 10.1109/TTHZ.2011.2159649 makes these THz lines important coolants for many phases of

TABLE I above ground state) are much less than the ionized gas temper- SELECTED THZ SPECTRAL LINES. atures , so that level excitation is very insensitive to gas temperature. However, neutral oxygen and carbon are found in neutral gas regions where gas temperatures are typically between 50 and 1000 K. Therefore, their line ratios are sensitive to gas temperature. The critical density of the neutral carbon lines are relatively modest so that the levels are typically thermalized at the densities found in interstellar gas clouds. As such, the [CI] line ratio has proven to be a good temperature probe, and the lines trace gas column density, hence mass. The THz frequency fine-structure lines are also excellent probes of the hardness of the ambient interstellar radiation fields. The vast majority 90 of stars in galaxies like the Milky Way are on the main sequence (MS) where they are fusing hydrogen into helium in their cores. Within the main sequence, stellar luminosities, are strongly dependent on stellar mass, . Due to the steep dependence of L on M, the most massive stars are not only the most luminous MS stars, they also have the shortest MS lifetimes, . Furthermore, the most massive MS stars have the highest surface temperatures, , so that they dominate the interstellar UV radiation fields. Therefore, presuming a stellar photospheric origin, a harder ambient UV radiation field indicates more massive stars on the main sequence, hence a younger stellar population. The fine-structure lines trace the the ISM and excellent probes of the physical conditions of the radiation field hardness, hence the age of the stars. It takes 14.5 gas clouds and tracers of the sources of heat, be it the radiation eV photons to form (equivalent 33 ,oraB0 fields from nearby stars or AGN, cosmic rays, X-rays or inter- star), and 29.6 eV photons to form ( 39 or stellar shocks. an O8 star). Therefore, the [NII]/[NIII] line ratios are strongly indicative of the hardness of the ambient interstellar radiation B. Ionized Gas Lines field (Fig. 2). With an ionization potential of 35 eV, takes even harder UV photons to form than . Therefore, the Ionizing hydrogen requires photons with energy greater [OIII]/[NII] line ratios are even stronger indicators of .The than 13.6 eV, so species that require more than 13.6 eV pho- [OIII] 88/[NII] m line is especially useful since the levels 1 tons to form will be found exclusively within HII regions that emit these lines have similar critical densities (Table I), so , and those with ionization potentials less that the resulting line ratio is very insensitive to density as well than 13.6 eV (O, C) will be found exclusively in neutral (hy- (Fig. 3). drogen) gas clouds. takes only 11.3 eV photons to form, but 24.4 eV to further ionize so that it is found both in neutral and C. Neutral Gas Lines ionized gas clouds. Fine-structure lines are excellent probes of gas density. Fig. 1 shows this sensitivity for the [NII] line pair.2 In astrophysical environments, as one moves away from the Within a species, two fine-structure lines will have different source of ionizing photons there is a transition from regions Acoefficients, hence different critical densities for where hydrogen is fully ionized (the HII region) to the neutral thermalization of the emitting levels. In the low density limit gas clouds beyond. While HII regions and neutral gas clouds , every collisional excitation leads to the emission have size-scales of the order 10 parsecs cm ,the of a photon, so that the line ratio is constant. In the high density transition between the two is fairly abrupt. It is roughly given by limit , the levels populations are thermalized, and the mean free path of a photon that is capable of ionizing H, or given by the Boltzman formula therefore yielding a second 0.01 pc for typical interstellar gas densities. Photons with en- constant line ratio. Between the low and high density limits ergies less than 13.6 eV escape the HII region and penetrate the the ratios vary strongly with gas density as first one, and then neutral gas clouds beyond where they can ionize elements with the other emitting level is thermalized. For the lines that arise ionization potentials less than 13.6 eV (e.g., C), and photodis- in ionized gas regions, the level excitation potentials (energy sociate molecules with dissociation energies less than 13.6 eV (e.g., CO, , ) forming a photodissociation region (PDR) 1An HII region is an interstellar gas cloud in which the hydrogen gas is fully ionized. (see [5]). The full depth of the PDR is often defined as the dis- 2In astronomical parlance, ionization states of a species, for example nitrogen, tance into the cloud to which the cloud physics and chemistry is are denoted by for the neutral, singly ionized, and doubly dominated by FUV eV photons. With this def- ionized states etc., while spectroscopic lines are denoted by NI, NII, NIII,… for inition the PDR can extend to visual extinctions, .Most lines arising from the neutral, singly ionized, and doubly ionized states etc… Brackets around a transition, e.g., [NII] indicate the transition is forbidden to of the far-IR fine structure line radiation arises from the warmer dipole radiation. regions closer to the surface within the “ ” zone, given by the STACEY: THZ LOW RESOLUTION SPECTROSCOPY FOR ASTRONOMY 243

of cm ,or 1 for a typical gas density of cm . Within the PDR, gas is heated by the photo-electric (PE) effect—whereby hot electrons are ejected from grains by FUV photons and transfer their excess kinetic energy to the gas before recombining with grains. The efficiency of this heating mechanism depends on the grain charge. As the charge on the grain increases, more of the next photon’s energy is expended in freeing the electron from the positive potential of the grain, leaving less excess kinetic energy for heating the gas, thereby lowering gas heating efficiency. Therefore, the gas heating efficiency goes down as the ambient radiation field strength goes up. However, this can be mitigated by larger gas densities that will increase electron-grain recombination rates, lowering the grain charge. The PE heating efficiency is typically 0.1 to 1%. Fig. 1. mto m line intensity ratio as a function of gas density (solid line). This ratio is a sensitive probe of gas density for HII The rest of the FUV energy goes into heating the grains which regions with electron densities between 20 and cm .Alsoshown then emit in the THz and infrared continuum. Within the is the m m line ratio as a function of gas density. zone, the gas is primarily cooled through collisional excitation This line ratio is used to discern the fraction of the observed line that arises from HII regions [2]. This plot assumes a gas phase abundance ratio of the ground state fine-structure levels of and O and subse- [1], and this ratio is corrected for ionization effects, see [2]. quent emission in their THz lines. To a lesser extent, the neutral carbon fine-structure lines and the mid-J CO rotational lines are also important coolants for these regions becoming more important at greater depths onto the cloud. The combination of the m , and [OI] (63 and m) fine-structure lines enables unique determination of PDR gas temperature and density. Since the combined luminosity of these lines gives the gas heating, and the luminosity of the THz dust continuum radiation is directly proportional to the luminosity of the ambient radiation fields, the ratio of the two yields the PE heating efficiency, hence strength of the ambient FUV radiation fields. It can be shown (see [6]–[9]) that Fig. 2. Ratio of the [NIII] mto[NII] mand m lines as a function of effective temperature of the ionizing star (assumed gas density for most astrophysical environments that apply over kilo-parsec cm ). This line ratio is a very strong diagnostic of the most massive (kpc) scales in galaxies, the [CII]/far-IR continuum ratio in of star on the main sequence with line ratios that grow by factors of 1000 when itself is an excellent indicator of the FUV field strength and can the stellar type changes from B2 to O7.5 stars (based on [3]). be used as an indicator of the physical size of the astrophysical source. A challenge with the [CII] line is that it arises from both ionized and neutral gas regions, so that its diagnostic potential can be problematic. Fortunately, the [NII] m line has the same critical density for excitation by electron impacts as [CII] so that the [CII]/[NII] m line ratio from HII regions is only a function of the assumed abundances within the HII region (see Fig. 1). Therefore, the [NII] m line strength yields the fraction of the observed [CII] emission that arises from ionized gas ([2]), and the [CII]/[NII] combination constrains the physical conditions of two major components (ionized and PDR) of the ISM.

D. The Importance of the THz Fine-structure Lines The THz fine-structure lines are bright. For star forming Fig. 3. Ratio of the [OIII] mto[NII] mand m lines for three galaxies, the brightest of these lines is typically the m ionized gas densities as a function of of the ionizing star. This ratio is [CII] line which can be the brightest single spectral line from extremely sensitive to and very insensitive to gas density, especially the [OIII] 88 to/[NII] mratio[4](basedon[3]). the galaxy as a whole, and typically accounts for between 0.1 and 1% of far-IR luminosity of the system ([7]). From the Milky Way galaxy this amounts to 70 million solar luminosities depth to which carbon ionizing photons can penetrate forming [10], [11], for nearby ultraluminous infrared galaxies, . This depth is typically determined by the extinction of these those with far-IR luminosities exceeding the [CII] photons by dust, and is roughly given by a visual extinction, line can have luminosities of ([12]), while in distant 3 , which corresponds to a hydrogen column density hyper-luminous galaxies , ten billion solar 244 IEEE TRANSACTIONS ON TERAHERTZ SCIENCE AND TECHNOLOGY, VOL. 1, NO. 1, SEPTEMBER 2011 luminosities of power often emerges in this single line ([9], detection, cf. [21]–[23], but their astrophysical detection was [13])! Other lines, in particular [OI] mandthe[OIII]lines hindered by (1) the strong telluric absorption from water vapor often are nearly equal in luminosity to the [CII] line and the which makes detection of all but the [CI] lines nearly impos- [NIII], [NII], and [OI] m lines are only a factor of 5 to sible from ground based telescopes, (2) the uncertain transition 10 fainter [14]. The [OI] and [CII] lines are especially bright frequencies of the major lines, and (3) the still emerging state of because they are the cooling lines for a dominant component of detector technology at this time. The primary issue was telluric the ISM, PDRs. Cooling lines are important as an interstellar absorption, which was largely erased with the advent of air- cloud must cool to enable it to collapse to form the next gen- borne and balloon borne telescopes in the 1970’s. At altitudes eration of stars. It is the extinction-free nature, and powerful above 12.5 km, one is typically above 99.8% of the atmospheric diagnostic utility of these lines, together with their intrinsic water vapor, opening up the THz windows, so that the THz brightness that makes their study so compelling, even from the lines were soon detected from these facilities. The [OIII], [OI], most distant sources in the Universe. and [CII] lines were first detected by Martin Harwit’s group at Cornell University using a grating spectrometer on the 30 E. Molecular Lines cm telescope in NASA’s Lear Jet Observatory [24]–[28]. The The brightest of the molecular lines in the THz regime are [NIII] line was first detected by the ESA/ESTEC and Uni- the mid-J rotational lines of CO. These lines probe the warm, versity College, London (UCL) groups using their Michelson dense gas immediately interior to the atomic regions in PDRs Fourier transform interferometer (FTS) on their balloon-borne and can be quite strong in molecular shocks. The run of line 60 cm telescope [29], [30]. The [NII] lines were first detected intensity with J constrains the physical conditions of the emit- with the FIRAS FTS on the COBE satellite [11] at about the ting gas. Several of the rotational lines can be optically thick, same time they were also detected by Edwin Erickson’s group so that radiative transfer effects must be considered. Typically at NASA Ames using a grating spectrometer on the 91 cm radiative transfer is handled within a “large-velocity gradient” telescope on NASA’s Kuiper Airborne Observatory (KAO) model. These models have degeneracies between optical depth [31]. Tom Phillips group at Caltech made the first detection of effects, and those of gas excitation (density and temperature), so the [CI] 492 GHz line with a heterodyne receiver on the KAO that it is important to measure low optical depth isotopologues [32], while the Betz/Genzel group at UC Berkeley obtained the of CO, most commonly transitions to break these degen- first detection of the 809 GHz [CI] line by using a heterodyne eracies. The absolute strength of the lines compared to the THz receiver on the UH 88” telescope [33]. Several of these lines continuum and fine-structure lines can reveal the source of heat. were also mapped with balloon-borne experiments including For example, strong CO lines and relatively weak THz lines in- the mapping of the inner Galaxy in [CII] by Okuda’s group dicates non-PDR origins for the CO emission: the gas may be at ISAS in Japan using their Fabry–Perot interferometer (FPI) heated by super-sonic shocks, cosmic rays or X-rays [15]–[18]. [34], and mapping of the Orion Nebula in several lines with the ESA/UCL Michelson interferometer [35]. These galactic F. The Importance of the THz Molecular Lines observations were soon followed up by extragalactic FPI work in the Townes/Genzel group at UC Berkeley and at MPE Stars form from dense molecular clouds. To enable sustained Garching (e.g., [7], [36]–[38]) and within Edwin Erickson’s collapse, clouds must cool through THz line radiation. Studies group at NASA Ames ([39]–[41]). All of these observations have shown that up to half of the molecular ISM in starburst3 of lines with wavelengths shorter than m were obtained galaxies is warm and dense [15], [16], [18]–[20] so that it is with direct detection spectrometers (gratings, FTS, and FPI). important to study this gas through its mid-CO cooling line The first THz molecular lines detected from external galaxies emission to understand the interplay between star formation were the bright mid-J CO rotational lines. These lines were first and the natal molecular clouds. For example, for some starburst detected using heterodyne receivers from galactic sources. The nuclei, supernovae blasts might compress the ISM leading to first mid-J CO line detected from an extra-galactic source was the formation of the next generation of stars so that the star- the CO(6–5) line at 690 GHz, detected from the nearby starburst burst is self-sustaining, while for others far-UV and cosmic ray nuclei of NGC 253, M82, and IC 342 using a Schottky-diode heating, and molecular flows may energize or disrupt the ISM based receiver [19]. The CO(7–6) (809 GHz) line was first de- making the starburst self-limiting [16]. Notice that since the low tected and mapped from an extragalactic source in the local Uni- lines have smaller excitation requirements, their line verse, NGC 253 using a direct detection spectrometer, SPIFI ratios are relatively insensitive to the physical parameters of the ([15] see below). (The line had previously been detected from warm dense gas. high redshift sources, e.g., [42]). The first extra-galactic mid-J line of detected was the 6–5 line, also detected from NGC III. ASTROPHYSICAL OBSERVATIONS 253, using the ZEUS grating spectrometer, ([16] see below). In 1996, ESA launched the Infrared Space Observatory, a 60 A. A Short History cm cooled telescope in Earth orbit that contained both a FPI The far-IR fine-structure lines discussed above were rec- and a grating spectrometer for THz spectroscopy. The space- ognized as important astrophysical coolants long before their based platform opened an un-obscured view of these lines, al- beit at wavelengths shorter than the long wavelength cut-off 3A starburst galaxy is one that is forming stars at such a high rate that it will exhaust is supply of star forming gas in a timescale short compared to the of stressed Ge:Ga photoconductors, near m. An example lifetime of the Universe. of ISO based fine-structure line science is outlined below. ISO STACEY: THZ LOW RESOLUTION SPECTROSCOPY FOR ASTRONOMY 245 also for the first time detected several molecules in absorption against the THz continuum from an extragalactic source [43], [44]. (OH and CH, and had previously been reported in absorption against the THz continuum of Sgr B2 [45]–[47]). The THz spectrum of the heavily imbedded nucleus of Arp 220 in particular shows many absorption lines (e.g., OH, ,CH, NH, ) which are excellent tracers of very small gas column densities, and gas phase molecular abundances [43]. As these rotational lines are permitted transitions, they are well coupled to the local radiation fields and have been used to constrain source geometry and the properties of the sources of the THz continuum radiation [44].

B. M82: A Case Study

There have been many excellent studies of nearby galaxies in the THz fine-structure lines since the discovery observations Fig. 4. ISO-LWS spectra of four infrared bright galaxies fromm mto outlined above. For brevity, we focus on studies of M82, the m showing most of the bright THz fine-structure line. Adapted from [50]. brightest extragalactic THz source in the sky, and the nearest starburst galaxy. In normal star forming galaxies like the Milky Way about 17% the ISM is in molecular clouds with size-scales formation in galaxies at redshifts beyond 2.2—when the Uni- of 20 to 40 pc, 60% is in PDRs, and 23% is in diffuse HII re- verse was less than 3 Giga years old. In this section, we briefly gions [48]. The PDR mass is dominated by the relatively diffuse describe the SPIRE results obtained on Mrk 231. atomic ISM, but a significant fraction 20 of the PDR mass Mrk 231 is a representative of the Ultraluminous Infrared is contained in dense PDRs formed on the FUV exposed sur- Galaxy (ULIRG) class of galaxies that were originally discov- faces of molecular clouds. In contrast, KAO studies of M82 that ered by the IRAS satellite. These galaxies emit most (up to included the [OI], [OIII], [NII], [NIII], and [CII] lines revealed 99%!) of their bolometric luminosity in the THz dust continuum an ISM fragmented by the powerful starburst into tiny 1 and by definition have THz dust continuum luminosities in ex- diameter molecular cloudlets [41]. These cloudlets are substan- cess of . The large THz to optical luminosity ratio means tially warmer and denser than Milky Way counterparts, and only their source(s) of radiant energy, be it stellar photospheres or half the mass of these cloudlets is molecular. The rest—nearly accretion disks enveloping super-massive black holes, are en- half the total ISM—is contained within dense PDRs on these shrouded in many magnitudes of extinction by dust. The ob- molecular cloudlet surfaces. This large mass fraction in dense scuring dust absorbs the optical radiation, heats up, and re-ra- PDRs is due to the intense FUV radiation field from the star- diates the energy in the THz continuum. Most 70 of the burst. This radiation field is about 1000 times stronger than that ULIRGs are thought to be powered primarily by star formation, of the solar neighborhood and extends over a 300 pc scale region with the rest powered primary by AGN [51]. Mrk 231 is from in the center of M82. The high pressure clouds are in near pres- the “mixed” class, so it will show characteristics of both. Fig. 5 sure equilibrium with ionized gas and a hot inter-cloud medium shows the remarkably rich SPIRE FTS spectrum of Mrk 231 that is 10–20% of the total interstellar gas mass. Subsequent [52]. Within this spectrum are 25 detected lines including lines studies based on ISO LWS spectroscopy (cf. Fig. 4) of these of CO from up to , 7 lines of lines strongly constrained the stellar mass function. It is best [53], 3 of , and one each of , ,andHF.The modeled as a 3 to 5 million year old instantaneous starburst with CO rotational spectrum up to is consistent with line a 100 solar mass upper limit to the initial mass function [49]. emission from warm, dense PDRs enveloping intense regions of star formation. At higher J, the spectrum flattens out with near uniform intensity. This flattening of intensity at the highest J is C. Recent Results: Herschel and Markarian 231 predicted for an X-ray dominated region (XDR)—a molecular ESA launched the 3.5 m aperture passively-cooled Herschel region whose primary source of heat is intense X-ray illumi- Space Observatory (HSO) on May 14, 2009. It is expected to op- nation [54]. The XDR is very likely associated with the “con- erate until its cryogen supply runs out sometime in early 2013. fining torus” of molecular gas thought to envelope the AGN, Herschel is dedicated to far-IR and submm wavelength photom- and the source of X-ray radiation is most likely the AGN itself. etry and spectroscopy and is performing at or beyond expecta- The strong and line emission strongly support the tions in nearly every mode. Of relevance to the discussion here XDR heating mechanism. This is the first direct detection of the are the Photodetector Array Camera and Spectrometer (PACS) hot molecular torus long thought to circulate about the super- and the Spectral and Photometric Imaging Receiver (SPIRE) massive black holes that power AGN. spectrometers operating in the 55 to 210 and 194 to m spectral regions respectively (see Section V). These direct de- D. Recent Results: The Early Universe tection spectrometers have been enormously successful deliv- The COBE satellite discovered that integrated over the his- ering a wide variety of science from studies of the planets to star tory of the universe, half of the photospheric emission from stars 246 IEEE TRANSACTIONS ON TERAHERTZ SCIENCE AND TECHNOLOGY, VOL. 1, NO. 1, SEPTEMBER 2011

Fig. 6. ZEUS/CSO detections of [OIII] m, [CII] m, and [NII] m ﬁne-structure lines from high redshift systems [63], [9], [4].

detection of [CII] from a high z galaxy not association with a quasar was made from MIPS J1428 at using the ZEUS spectrometer [8]. Here the ratio is large, compa- rable to nearby starburst galaxies, and when combined with the CO(1–0) line, indicates the ISM in MIPS J1428 contains molecular clouds with similar gas densities and exposed to similar radiation fields as those found in M82. But, since MIPS J1428 is forming stars at the rate of 1800/year compared to 5 Fig. 5. SPIRE FTS spectrum of the ultraluminous galaxy Mrk 231 [52]. Red, for M82, the starburst must be enormous in this source, with an blue, magenta, and cyan lines are CO, H O, OH ,andH O , respectively. estimated diameter of 3–8 kpc, essentially involving the entire galaxy. This first detection was followed up by a [CII] survey containing 12 detections of galaxies in the 1 to 2 interval in is absorbed by dust in their nascent molecular clouds [55], [56]. the line quadrupling the numbers of high z systems then re- The heated dust then reradiates its energy in the FIR, with power ported [9] (Fig. 6). Within this survey, those systems that derive typically peaking in a broad band centered near m. most of their energy from star formation have a lu- Much of this cosmic infrared background was subsequently re- minosity ratio eight times that of those that derive most of their solved by large and sensitive telescopes into several classes of energy from AGN activity, so this ratio picks out systems dom- luminous star forming galaxies that emit primarily in the FIR inated by star formation. As for MIPS J1428, the star formation to submm bands 30 m mm ,mostofwhichlie dominated systems have starbursts similar in intensity to that of at redshifts beyond 1 or look-back times beyond 7.7 Giga years M82, but the starbursts extend over kiloparsec scales—very un- (Gyr) [57], and indeed, the peak of star formation activity per like the confined (few hundred pc scales) found for starbursts in unit co-moving volume occurred between redshifts 1 and 3 (2 local starburst or ULIRG galaxies. Essentially the entire galaxy to 6 Gyr after the Big Bang) at a rate 10–20 times the current is erupting in a super-massive starburst. Clearly the mode of star value [58]. Continuum surveys are the means of discovery for formation is quite different at early times than it is today. these distant galaxies, but spectroscopy is the only method for The THz fine-structure lines of [OIII] are now reported from unraveling the physics of the interstellar medium and the prop- high z systems. The m line was detected first from two erties of the stellar populations. Since these systems are highly AGN/starburst systems at high redshifts using the ZEUS spec- obscured, FIR/submm spectroscopy is key to these studies. trometer on the CSO [63] (Fig. 6). For SMM J02399 , The [CI] lines were the first THz fine-structure lines detected the line was used together with the THz continuum to estimate at high redshift, detected from the lensed Cloverleaf quasar at the age of the starburst and most massive star on the main se- [42], [59]. These lines, together with CO lines, were quence. For APM 08279 it was found that the line used to show that the molecular disk has surprisingly modest could arise from either the narrow line region of the AGN, or mass within this system. The brightest THz fine-structure from a starburst headed by O7.5 stars. Very recently, the [NII] line, the [CII] line was firstobservedathighredshiftfromthe m line was detected for the firsttimeathighz,fromSMM very high redshift quasar, SDSS J02399 and the Cloverleaf [4] (Fig. 6). The [OIII]/[NII] line [60]. The line was subsequently detected in two other high ratio tightly constrains the radiation field (Fig. 3), and it is found quasar dominated systems, each of which show a that SMM J02399’s current stellar population is dominated by low ratio reminiscent of that found in some ULIRG O9.5 stars, indicating a 3 million year old starburst. galaxies in the local Universe [61], [62]. In local ULIRGs this The [OIII] m line was detected by PACS on the HSO low ratio is interpreted as either intense FUV fields lowering the from MIPS J1428 , and IRAS F10214 luminosity ratio since [CII] line saturates, while [64]. The line is compared with the THz continuum to show that continuum does not, or due to presence of additional non-PDR the former source is similar to local starburst galaxies, while the origin for the FIR continuum [12]. latter is similar to local ULIRGs. The first high z detection of The first detection of a high z [CII] line from the [OI] m (from MIPS J1428) was also reported in this paper, epoch of peak star formation in the Universe, and also the first confirming the [CII] based conclusions [8]. STACEY: THZ LOW RESOLUTION SPECTROSCOPY FOR ASTRONOMY 247

IV. DIRECT DETECTION SPECTROSCOPY where or 2 depending on whether the receiver is config- ured for single-side-band or double-side-band detection. Clearly if the background is large, and the receiver temperature is small, It is very challenging to achieve very high spectral resolving the effect of the receiver temperature on the overall sensitivity powers (RP) with direct detection systems in the THz regime. is small. This would be the case for a receiver at 100 GHz, For instance, in a single pass configuration, a grating spectrom- with noise temperature 3 15 , operating from the eter with 10 at 1 THz m needs ground where the combined emissivity of the telescope and the an effective delay path, 10 m . sky might be 15%, i.e., . However, Even within a multiple pass system, like a Fabry–Perot with fi- in space, where can approach 2.73 K, and at THz frequen- nesse 30 the path is still a challenging 50 cm. Therefore, the cies, where 3 to to 720 K, the receiver noise only reasonable way to achieve these very high RP at THz fre- can easily dominate that of the background. quencies is with heterodyne detection techniques. However, for In principle, the sensitivity of a direct detectionsystemis the modest RP required for extragalactic spectroscopy (1000), only limited by the statistics of the photon arrival rates. For delay paths are modest 15 cm so that direct detection spec- the case of low photon occupancy , which ap- trometers are straightforward to construct. plies in the optical band, the noise follows a Poisson distribu- In an incoherent, or direct detection spectrometer the energy tion, so that the detection of N photons, has an associated noise, of the incoming photon is used to create a signal either by . However, in the THz regime, 1, so that the heating a detector (as in a bolometer) or in causing an electron effects of photon bunching must be included. The noise equiv- to make a quantum transition from a valence to conduction band alent flux, , of a background lim- in a semi-conductor (as in a photoconductor). A coherent, or ited performance (BLIP) spectrometer with cold transmission indirect detection system relies on the wave nature of light. The (emissivity 0), operating on a telescope with collecting area, advantage of a coherent receiver is that the spectral RP can be A, efficiency , at temperature T, looking through a sky with made extremely high, and the preservation of phase makes them transparency , at temperature T, using spectrometer pixels easy to use for aperture synthesis. The systems we compare to that have point source coupling , and detective quantum in the THz band are heterodyne receivers, where the THz wave efficiency (DQE) , is given by [69] from the astronomical source is beat against a local oscillator to mix the signal down to gigahertz frequencies where it is efficiently detected in a square law mixer—one who’s output signal power is proportional to the square of the input field strength—such as an SIS mixer. Coherent detection preserves where ,and are as before, is the beam solid angle, and photon phase and requires signal amplification before detection. is the product of the warm emissivity , , This results in an uncertainty in photon occupation number of the spectrometer cold transmission , and the mode occupation one photon/Hz/sec. This “quantum noise” when expressed in number, . The effective emissivity of terms of temperature of a radiation field that produces the same the background , is roughly given by noise, is given by ,whereh .Thefirst factor of 2 in this expression arises due to and k are Planck’s and Boltzman’s constants respectively. The chopping losses of the telescope, while the second factor of 2 is quantum noise is unavoidable, and imposes a sensitivity floor due to the detection of both polarizations of light. The equivalent for the receiver noise temperature of a heterodyne receiver: expression for the heterodyne system is . The best modern receivers come within a factor of of the QN limit at low frequencies. For example, the best double-side-band (DSB) receiver temperatures, are: @ 345 GHz [65], and 75 K at 670 where is the effective fraction of the power in the main GHz [66]). At frequencies above 1.2 THz, the best receiver beam that couples to a point source due to the edge taper of the temperatures are 1000 @ 1.3 THz [67], and beam, and the other parameters are as above. Equating the two at 2.55 THz [68]. expressions and using our equation for above, we have Arguably, one of the most exciting applications of moderate RP THz spectroscopy today is the detection of the THz fine- structure lines from high redshift galaxies. This entails maxi- mizing point source sensitivity over broad bandwidths. Here we The factor of arises since we assume diffraction lim- compare the relative utility of coherent and incoherent receivers ited, Gaussian beams, where ,and for this work, and demonstrate a compelling case for direct de- .Here is the beam full-width-at-half maximum, tection at THz frequencies. In terms of the receiver temperature and D is the diameter of the telescope primary mirror. We use the in the presence of background with equivalent noise tempera- ZEUS and ZEUS-2 spectrometers as an example (see below). ture , the rms noise temperature at the front end of a co- These spectrometers have 35 , bolometers with 0.9, herent receiver, in a bandwidth ,inintegrationtime, and 0.7. Referring to a single sideband receiver is given by then for BLIP at 850 GHz at an excellent ground based site 58 and assuming ,aco- herent single-side-band receiver would need to have a noise temperature of 16 K—a factor of 2.5 below the quantum noise 248 IEEE TRANSACTIONS ON TERAHERTZ SCIENCE AND TECHNOLOGY, VOL. 1, NO. 1, SEPTEMBER 2011 limit—to be equivalent to a background limited grating spec- ever, a spectrally multiplexed grating spectrometer can have N trometer for point source detection. The ZEUS spectrometer de- detectors in the dispersion direction, so that it need not spec- livers sensitivity equivalent to single side band receiver temper- trally scan. For this case then, the grating spectrometer is more atures 10 at RPs [70]. With modern bolome- sensitive than the FTS by . If, however, the detectors are ters, it is straight-forward to achieve BLIP in the THz bands for not background limited—i.e., the noise is independent of band- RPs , from the ground and airborne facilities so that width—the sensitivities become equal again. In this case, the direct detection systems are the spectrometers of choice for de- FTS becomes a very interesting choice since it delivers the same tecting broad spectral lines. N resolution element spectrum as the grating spectrometer, but with just one detector, not N so that it provides a much more ef- V. D IRECT DETECTION THZ SPECTROMETERS IN USE TODAY fective use of what is often the rarest commodity, the numbers In this section we describe four current state-of-the-art of pixels. spectrometers in use at THz frequencies today, including a Fabry–Perot interferometer (SPIFI), a long-slit grating B. SPIFI—A Bolometer-Based Imaging Fabry-Perot spectrometer (ZEUS), an image slicing grating spectrometer The South Pole Imaging Fabry-Perot Interferometer (SPIFI) (PACS), and a Fourier transform spectrometer (SPIRE). Each is the first direct detection imaging spectrometer for use in of these spectrometers performs close to the fundamental the submillimeter bands [69], [73], [74]. Unlike previous limits, and due to their unique architecture, has a unique niche THz astronomical spectrometers that used either heterodyne of scientific excellence. techniques, or photoconductors, SPIFI employed bolometers A. Comparisons Between Direct Detection Spectrometers as detectors for the first time. Bolometers have several clear advantages over photoconductors: (1) Ge:Ga photoconductors The choice of direct detection spectrometer strongly depends have DQE’s of the order 3 to 26% [75], [76]. In contrast, it on the science at hand. An FPI or an FTS have much larger is easy to design a bolometer with 50% DQE over very large luminosity-resolution products at a given R than grating spec- bandwidths, or by using tuned backshorts, DQE over trometers that require entrance and exit slits [71]. The FPI/FTS broad 3 bandwidths [77]. (2) Photoconductors de- are also readily adapted to imaging spectroscopy so that within tect photons by discretely generating photoelectrons (or holes). limits given by the angular divergence of rays in the beam, The generation and recombination of these photoelectrons gen- large, 2-dimensional focal plane arrays of detectors are easily erates shot noise, commonly termed generation recombination implemented into these spectrometers creating spectroscopic (GR) noise. Bolometers are thermal devices so that they do not imagers. For detection of a single isolated line over broad re- suffer from GR noise. Therefore, for a given DQE, a bolometer gions, the FPI will win over an FTS since, in a BLIP environ- is more sensitive than a photoconductor. (3) Bolometers ment, the narrow band of the FPI will deliver much higher sen- are inherently broadband with nearly constant DQE over this sitivity than the broadband FTS (see below). If, however, one band. Unlike photoconductors, bolometers do not have long wishes to detect many lines over a very broad range, this sen- wavelength cut-offs due to minimum energy requirements for sitivity advantage of the FPI becomes small, and an FTS be- promotion of electrons from the conduction to valence bands, comes competitive. However, in terms of raw sensitivity for or poor short-wavelength performance due to strong frequency point source detection, a spectrally multiplexed grating spec- dependence of ionization cross sections. The photoconductor trometer is the instrument of choice. with the longest wavelength cut-off is stressed Ge:Ga, which Let us compare a monochrometer (FPI or grating spectrom- has a long-wavelength cut-off near mor1.43THz.The eter) to an FTS of the same RP (see also [72]). We desire to combined sensitivity advantage of bolometers as detectors over obtain a spectrum with overall bandwidth B, split into N spec- photoconductor goes as NEP , and can be tral resolution elements, . Suppose both large. As a working demonstration, under similar background instruments have BLIP with noise per resolution element in conditions, SPIFI at 1.5 THz m is a factor of more the monochrometer given by . The FTS takes in the en- sensitive than background limited spectrometers that employed tire bandwidth, B at the same time, encoding the power of all stressed Ge:Ga photoconductors as detective devices [2], [31], spectral elements into each step of the interferogram. Since the [75]. FTS detector sees the full bandwidth, the noise per unit time is SPIFI is an imaging Fabry–Perot interferometer designed to larger than that of the monochrometer by work in the 850 GHz window available to the JCMT on Mauna . However, if the monochro-meter is Kea, and the 1.5 THz window available to the AST/RO tele- an FPI, one has to scan N resolution elements to deliver the scope at South Pole [69], [73], [74] (Fig. 7). SPIFI employs equivalent spectrum as the FTS does in a single inteferogram. free-standing metal mesh mirrors in its FPI etalons, which typ- Therefore, the total time to deliver the same spectrum is N times ically deliver reflective finesse 25 , and transmissions longer, so that N interferograms are obtained in the same time 70 . SPIFI’s RP is fully tunable through changing the as one FPI scan. In this case, then the noise of the FTS spec- order of the high-order FPI (HOFPI), and can be changed from trum goes down by , and the two systems are equal.4How- to in a few minutes while the instrument is 4Note that we have ignored the factor of 2 encoding losses of the FTS in this mounted on the telescope and operational. SPIFI achieves high argument. This loss is because the average signal of the interferogram is half the spectral purity through the use of a low-order FPI (LOFPI) to se- white light signal. However, the FPI will likely have smaller transmission than the FTS due to etalon absorption or the extra filtrationneeded,sothisencoding lect the desired order of the HOFPI, and a fixed bandpass filter loss is at least in part cancelled in the comparison. to select the desired order of the LOFPI, enabling operation of STACEY: THZ LOW RESOLUTION SPECTROSCOPY FOR ASTRONOMY 249

Fig. 7. Optical layout for SPIFI imaging Fabry–Perot [69].

Fig. 8. Optical layout for ZEUS [70]. The grating is in near Littrow configura- the LOFPI in orders as high as 10 to 20. The detective devices tion. in SPIFI are a 5 5 array of bolometers from Goddard Space Flight Center (GSFC), that are held at 60 mK in an adiabatic de- and delivers a RP . This RP is near the diffraction limit, magnetization refrigerator (ADR). SPIFI was deployed 6 times and the plate scale was selected so that most 50 on the 15 m JCMT. Scientific highlights from these runs in- of the flux from a monochromatic point source is coupled into cluded the first detection and mapping of the CO(7-6) line from a single pixel. Since extra-galactic line widths are typically NGC 253, and the first large-scale map of the CO(7-6) and [CI] , this choice of RP, and pixel size optimizes sensi- lines from the Galactic Center circumnuclear ring [15], [78], tivity for broad lines from point sources—e.g., distant galaxies. [79]. ZEUS employs a 1 32 pixel array of planar bolometers from SPIFI was also deployed on the 1.7 m AST/RO telescope at GSFC with backshorts tuned to m resonance that delivers South Pole over the 2004 and 2005 Austral winter. The first quantum efficiencies over most of the operational band. season was cut short due to an unexpected rapid depletion of ZEUS is run with order selecting bandpass filters immediately the liquid helium supply for the base, but the second season was above the detector array: typically half the array operates in the quite successful. SPIFI was used in both the 859 GHz and 1.5 m and half in the m windows. With each pixel corre- THz telluric windows. The highlight of the deployment was the sponding to a resolution element, the instantaneous bandwidth detection and mapping of the [NII] m line from the Carina of ZEUS is typically to 14 GHz. ZEUS is very close to star formation region [2], [80]. This was the first detection of this background limited, coming within a factor of 1.2 and 1.3 of astrophysically important line from the ground, and its strength, BLIP at 350 and m respectively. This performance figure when compared with the ISO [CII] and [NII] mmapping corresponds to and 35 K respectively in the of the nebula demonstrates that the HII regions enveloping the centers of the 350 and m grating orders. Carina Nebula are low density 30 cm , and that only ZEUS has been a regular on the 10.4 m CSO telescope since 30% of the observed [CII] line radiation comes from the ionized spring of 2006. Highlights of ZEUS/CSO include the detection medium (Fig. 1). Most (70%) of the [CII] radiation arises from of the line from the starburst nucleus of NGC PDRs. 253—the first detection of the line from an external SPIFI was close to background limited in both bands of oper- galaxy, and the first detection of any transition greater ation: within a factor of 2 at 850 GHz [73], and within a factor than from a source beyond the Magellanic clouds [16], of 1.4 at 1.5 THz [74]. The equivalent single-side-band receiver and the several high redshift firsts mentioned above including temperature was 310 (40KDSB)at850GHz, (1) the first detection of the a THz fine-structure line of [OIII] at and 610 (190 K DSB) at 1.5 THz, which com- redshifts beyond 0.05 [63]; (2) the first detection of the [CII] line pare quite favorably with the best heterodyne systems today from the epoch of maximum star formation in the early Universe [65], [81]. [8]; (4) the first survey of the [CII] line at high redshift [9]; and (5) the first detection of a THz [NII] line from a source at redshift C. ZEUS/ZEUS-2: Bolometer Based Grating Spectrometers beyond 0.05 [4] ZEUS is an echelle grating spectrometer currently config- ZEUS-2 is a multi-color, multi-beam version of ZEUS. ured for operation in the 350 and m windows available The final instrument utilizes two TES bolometer SQUID on Mauna Kea [70]. ZEUS employs a 38 cm long R2 echelle multi-plexed arrays from NIST [82]. The first is tuned with grating blazed for m in 5th order (Fig. 8). As such, the a back-short to m, and consists of 280 pixels ar- 5th and 4th orders of the echelle are well matched to the 350, rangedina format for use in and m (860 and 670 GHz) telluric windows respectively the 300, 350 and m windows. The second consists (Fig. 9). The ZEUS grating is operated in near-Littrow mode, of two sub-arrays. The firsthas219pixelsarrangedina 250 IEEE TRANSACTIONS ON TERAHERTZ SCIENCE AND TECHNOLOGY, VOL. 1, NO. 1, SEPTEMBER 2011

D. PACS

PACS is one of the three science instruments that operate on ESA’s 3.5 m Herschel Space Observatory [83]. There are two modes to PACS, an imaging photometer mode, and an integral field spectrometer mode that delivers 16 pixel, Nyquist sampled 1700 spectra for each beam over a 5 5 beam (47” 47”) field of view. The integral fieldisobtainedbyslicingthetele- scope image plane into 5 rows of 5 beams each with mirrors that direct and map each row independently onto a long grating entrance slit (Fig. 11). A long slit spectrometer is thereby trans- formed into a compact footprint imaging spectrometer. The detector arrays for PACS are two 16 25 pixel arrays of stressed and unstressed Ge:Ga photoconductors that are sensitive to photons between 105–210 and 57– m respectively. The 30 cm long near Littrow mode grating is operated in 1st, 2nd and 3rd Fig. 9. Echelle orders (n) of ZEUS-2 superposed on telluric transmission plots order covering 105–210, 72–105, and 55– m respectively. for CCAT (best10%, 25%, and 50% of nights), at the ALMA site (best 50% of The first order is separated from the 2nd and 3rd orders by a nights), and at the CSO (best 50% of nights). CSO best 25% is similar to ALMA best 50%. ZEUS only accesses and 5 orders. dichroic beam splitter and sent to the stressed Ge:Ga array, while the 2nd and 3rd orders are passed to the unstressed array. Along the short wavelength path a filter wheel enables selection of 2nd or 3rd orders. The detector arrays in PACS deliver peak DQE near 26% [76], resulting in , 1 hour detection limits of about 3E- . These values are similar to those of ZEUS on CSO, so that there is a natural synergy between the two systems: redshift 1 to 2 galaxies detectable in [CII] with ZEUS are likely detectable in the [OI] mor[OIII](52or m) lines with PACS greatly enhancing the science yields of both systems. The un-obscured views from space, combined with the great sensitivity of PACS has already lead to a number of exciting scientific results, including: (1) the first detection of the [OIII] m and [OI] m fine-structure lines from high redshift galaxies [64]; (2) the first survey of local starbursts, Seyfert (AGN), and IR luminous galaxies in the ensemble of far-IR Fig. 10. (left) ZEUS-2 grating, optics, and cryogenics. (right). ZEUS-2 focal fine-structure lines showing changes in the far-IR line to far-IR plane with grating tuned to simultaneous long-slit spectroscopy in the CO(7-6), ,[NII]and[CI]370and m lines [82]. continuum ratio suggesting a shift to a higher efficiency mode of star formation in the most luminous systems [84]; and (3) the first evidence for massive molecular outflows in ULIRG galaxies that may quench their star formation activity [85]. format, and is tuned with a backshort to m for use in the 200 and mtelluric windows. The second sub-array consists of 56 pixels arranged in a format, and is tuned with a E. SPIRE backshort to m. The arrays should achieve in all bands. ZEUS-2 uses commercially available dual stage SPIRE is the second direct detection instrument on the HSO ADR and pulse tube cooling systems that not only eliminate [86]. Like PACS, SPIRE contains both imaging photometer, the need for expendable cryogens, but also lower the operating and imaging spectrometer modules. The spectrometer is an temperature of the cold head from 210 mK (with the ZEUS FTS in a Mach–Zehnder configuration. The FTS contains twin dual stage system) to 100 mK, ensuring greater margin on interferometer arms modulated by the same scan mechanism, detector sensitivity. so that spectra are simultaneously obtained from two different ZEUS-2 is versatile. By using a filter wheel, the 9th through bolometer arrays that are arranged to spatially overlap on the 2nd orders of the echelle are available for broadband spec- sky (Fig. 12). The first array covers the 194– mbandwith troscopy (Fig. 10). For low redshift, resolved objects ZEUS-2 37 bolometers, and the second covers the 303– mband delivers simultaneous spectroscopic imaging of five important with 19 bolometers. The bolometer arrays consist of discrete astrophysical lines: the [NII] m, [CI] 609 and m, “spider-web” bolometers fed by single mode conical feed CO(7-6) and lines (Fig. 10). ZEUS-2 is nearly horns. These open architecture arrays deliver very low NEPs at ready for first light on the CSO in the fall of 2011, and the base temperature of 0.3 K delivered by the refrigerator should deliver BLIP with equivalent receiver temperatures of plus exceptionally low susceptibility to cosmic ray hits [87]. in all bands. The FTS resolution element is fully tunable from 2– cm STACEY: THZ LOW RESOLUTION SPECTROSCOPY FOR ASTRONOMY 251

Fig. 11. (left) Mapping the focal plane image onto a long slit within the PACS spectrometer. (right) [OI] m and [OIII] m ﬁne-structure lines detected by PACS from the starburst galaxy MIPS J142824 [64].

VI. FUTURE PROSPECTS There are many new astrophysical discoveries being made right now using low resolution THZ spectroscopy boosted by both developments in ground based instrumentation, and the spectacular success of Herschel in space. The future is even more exciting. The SOFIA facility is just now undergoing its first science flights, the enormously powerful ALMA facility has just released its call for first science proposals, and looming on the near horizon are the 25 m CCAT telescope, and the 3.5 m SPICA space mission. We briefly discuss the scientific prospects for each of these facilities

A. SOFIA Fig. 12. (left) SPIRE interferometer optical path (right) close-up of a SPIRE spider-web bolometer showing the mesh architecture of the absorber, and the The Stratospheric Observatory for Infrared Astronomy thermometer. The thermometer is m in size [86], [87]. (SOFIA) contains a 2.5 m telescope in a modified Boeing 747 SP [90] (Fig. 13). Like CCAT, SOFIA is a readily accessible evolving observatory under which we may develop the next (unapodized), so that the highest RP varies from 1300 and 370 generation of instrumentation, taking advantage of the latest from the shortest to longest wavelengths. technologies. A THz image slicing spectrometer, FIFI-LS is The unique SPIRE niche is very broadband moderate reso- developed for SOFIA, and will be deployed in 2012 [91]. lution spectroscopy, underscored by the un-obscured vision of FIFI-LS is similar in many respects to PACS, but the SOFIA the HSO in space. SPIRE therefore has observed a variety of facility is better optimized for mapping experiments, so that lines for the first time, or the first time from an external galaxy for FIFI-LS/SOFIA we may expect, for example, large format (Fig. 5). The broad bandwidth also enables simultaneous mea- mapping programs of nearby resolved galaxies revealing the surements of many rotational transitions of CO from galaxies, roles of spiral arms and bars in compression of the ISM and ig- characterizing both PDRs and XDRs, and enabled the first nition of the next generation of star formation in spiral galaxies. detailed studies of water line emission from external galaxies New THz spectrometers will be deployed in future years on [53]. SPIRE has performed far better than pre-launch expec- SOFIA including heterodyne arrays, and bolometer based tations, delivering the requisite sensitivity to detect redshifted direct detection systems enabling new science. After Herschel fine-structure lines at as well [88], [89]. cryogens expire, and before SPICA is launched, SOFIA will be 252 IEEE TRANSACTIONS ON TERAHERTZ SCIENCE AND TECHNOLOGY, VOL. 1, NO. 1, SEPTEMBER 2011

Fig. 14. (left) CCAT concept. (right) Simple rotating periscope design for con- verting a long slit, or stacked Z-Spec-like spectrometer into a multi-object spectrometer. Each beam is coupled into the focal plane by the periscope design, and Fig. 13. SOFIA is a Boeing 747 SP modiﬁed to carry an open port 2.7 m tele- assigned a “patrol-area” of the focal plane [95]. scope (chopped aperture 2.5 m), that enables routine access to the highest THZ frequencies [90].

of THz frequency Z-Spec-like spectrometers thereby creating a the only facility from which high frequency THz observations multi-object system. can be made. Due to its large aperture, exceptional surface, exceptional site, and the use of very broad bandwidth direct detection multi- B. CCAT band spectrometers, CCAT can detect THz lines from distant galaxies at the rate of tens per hour. Once detected, particu- CCAT is a 25 m submm telescope under design by a consor- larly interesting sources can be delivered to ALMA (see below) tium of universities led by Cornell University that includes Cor- for follow-up observations with its exquisite velocity resolved nell, Caltech, the Universities of Colorado, Bonn, and Cologne, imaging capabilities. and a consortium of Canadian Universities [92]. CCAT is to have an exceptionally good surface ( mRMS),andtobe C. SPICA sited near the peak of Cerro Chajnantor in the northern Atacama desert, at an elevation of 5600 m so that CCAT will deliver ex- SPICAisa3.2mtelescopecooledtobelow6Ktobe ceptional sensitivity in the submm bands. Wide-field surveys in launched by the Japanese space agency, JAXA as early as the THz continuum with CCAT promise to nearly fully resolve 2020 [96]. With its large, very cold aperture in space, SPICA the THz background radiation into its constituent sources. In a promises to deliver unrivaled sensitivity in the high frequency 2 year survey, CCAT will detect hundreds of thousands of high THzbands.AtpresenttheSAFARIspectrometertobedeliv- redshift galaxies enabling statistically significant studies of star ered by ESA is the baseline THz spectrometer [97]. SAFARI formation to redshifts approaching 10—looking back into time covers the 30– m band with tunable RP to 10,000, to within 500 million years of the Big Bang. These continuum Mach–Zehnder imaging FTS, and should deliver sensitivities surveys will discover the sources, but to fully understand the 20 to 30 times better than PACS/Herschel. There may also be an star—and galaxy— formation process in the early Universe will instrument from NASA on SPICA. A concept based on a com- require direct detection spectroscopy in the various lines as out- bination of cross-dispersed free space spectrometers like the lined above. This is surprisingly straight-forward. The observed Spitzer IRS modules [98], and wave-guide spectrometers both brightness of the [CII] line in redshift 1–2 star forming galaxies operated with TES bolometers, called BLISS is under study [8], [9] is consistent with a [CII] to underlying THz dust con- [99]. BLISS will operate in the 38 to m regime. The goal tinuum ratio of 10:1. Since continuum sensitive cameras will is for BLISS to come within a factor of two of the background have bandwidths of 10%, or RP , and grating spectrom- limit determined by cosmic backgrounds at a RP .This eters optimized for line detection have RP , the sensi- would yield point source sensitivities 300 times better than tivity ratio is . Therefore, it is as easy to de- Hershel, opening up the most distant galaxies for spectroscopic tect the [CII] line as it is the underlying continuum, provided the study in the THz bands. For instance, BLISS/SPICA should source redshift falls into the telluric windows. CCAT should de- be able to detect the [OI] m line emission from a ULIRG tect 1000’s of galaxies per square degree in [CII] and other THz galaxy at ( , 2 hours), or to within 1 Gyr of the Big fine-structure lines. For detection of sources with unknown red- Bang. shifts, very large bandwidths are desired. This can be achieved with free space grating spectrometers in low (1st or 2nd) order, D. ALMA or a THz version of Z-Spec. Z-Spec is a mm wave direct detec- The Atacama Large Millimeter/Submillimeter Array tion spectrometer that achieves very broad detection bandwidth (ALMA) is a large interferometer being built on the Chaj- using a curved diffraction grating in a parallel-plate waveguide nantor plateau in the Atacama Desert in northern Chile by an [93], [94]. Either system can deliver detection international collaboration including Europe, East Asia, and with good optical efficiency. Since sources will be detected North America in cooperation with the Republic of Chile. A at the confusion limit on the sky a multi-object direct detection primary science goal is detection of the [CII] line from a Milky spectrometer is of importance. Simple concepts for feeding mul- Way-like galaxy at 5 in less than 24 hours observing time. tiple beams into spectroscopes exist. For example, a quasi-op- When completed ALMA will consist of 54-12 m and 12-7 m tical design involving twin elbowed periscopes [95] (Fig. 14), high surface accuracy antennas linked for interferometry. Its could beams into a long slit spectrometer, or a stack exceptionally high (5000 m elevation) and dry site enables STACEY: THZ LOW RESOLUTION SPECTROSCOPY FOR ASTRONOMY 253

TABLE II these sources. How does the stellar population of IR luminous ALMA RECEIVER BANDS. galaxies evolve over time? With its small beam, CCAT sources will have positional coordinates adequate for detailed follow-up with the high sensitivity and high spatial resolution of ALMA. Do the THZ lines arise from the AGN/torus (localized emission) or from starbursts (extended)? How extended are the starburst? Are they stimulated by galaxy-galaxy collisions? Finally, the sensitivity of SPICA enables detection of many shorter wavelength lines that have critical diagnostic features. For example, SPICA enables detection of the [OIV] mline routine observations in the short submm bands. The ALMA from these high redshift sources. This line, when compared facility is designed to operate from 31.3 GHz to 950 GHz, with the THz [OIII] lines is a definitivemeasureofAGN sub-divided into 10 bands based on technology and telluric activity. 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He received eration submillimeter grating spectrometer for exploring distant the B.A. degree in physics and mathematics at Grinnell College, Grinnell, IA, in galaxies,” in SPIE Conference Series, Jul. 2010, vol. 7741, pp. 1979, and the Ph.D. degree in astronomy from Cornell University, Ithaca, NY, 77410Y-77410Y-14–77410Y-77410Y-14. in 1985. [83] A. Poglitsch et al., “The photodetector array camera and spectrometer He was first a Postdoctoral Associate, then a Research Physicist in the Physics (PACS) on the Herschel Space Observatory,” Astron. Astrophys., vol. Department at UC Berkeley from 1985 until 1991. In 1991 he obtained a fac- 518, pp. 9–20, Jul. 2010. ulty position in the Astronomy Department at Cornell University, where is now [84] J. Gracia-Carpio et al., “Far-infrared line deficits in galaxies with ex- Professor of Astronomy. treme ratios,” Astrophys. J., vol. 728, pp. L7–L11, Feb. Prof. Stacey is a member of the American Astronomical Society and the In- 2007. ternational Astronomical Union.